MSMG ToolKit is gradually becoming similar to NTLite and WinReducer using ToolkitHelper.exe for manual removal of files and reg keys instead of using Dism. However it is not fully opensource, just ToolkitHelper.exe does not have your source code available. This looks promising. Although not intended for any user: https://forums.mydigitallife.net/threads/a-few-testers.77134/